The prior art includes a number of systems for radiographic imaging using neutrons. Typical applications include the inspection of aircraft parts. This method is more advantageous, in many respects, than X-ray inspection due to the fact that neutrons are easily absorbed by hydride compounds of aluminum which exist in corroded areas. Also, neutron radiography is superior to its X-ray counterpart in the inspection of hydrogenous zones. This allows accurate inspection of continuity of pyrotechnic materials in their containers.
Prior art neutron radiographic imaging systems have suffered from image degradation due to a number of factors. A primary factor is the inherent existence of fast/epithermal neutrons originating at a neutron source which constitutes noise. Extraneous gamma and X-rays from the source also contribute noise. Room scattered thermalized neutrons also contribute a component to detected noise. Finally, detected ambient radiation reduces the available signal-to-noise ratio of a detected image.
In conventional systems, the source of neutrons is moderated by thermalizing the neutrons in a material such as polyethylene. However, since the source and detection of such prior art systems work in a DC mode, the noise factors outlined above continue through a measurement cycle.
In the past, several hardware approaches have been employed to ameliorate the situation. For instance, heavy shielding has been employed for fast neutrons and gamma rays around a moderator component of a system. Further, heavy shielding and 90.degree. mirrors at the detector have been used. Clearly, this adds expense, complexity, and weight to a manufactured system.
The present invention is intended to increase the quality of a radiographic image by correspondingly increasing the signal-to-noise ratio of detection.